Get ready for the season’s first homeless action hero. Although to be fair, Jack Reacher is homeless by choice — a self-declared hobo, not a vagabond. The star of the best-selling crime thriller series by Lee Child is a bruising former military police officer who now lives off the grid, traveling by bus with nothing more than the clothes on his back.

In the books, one character describes him like this: “His lazy lopsided grin. His tousled hair. His arms, so long they gave him a greyhound’s grace even though he was built like the side of a house. His eyes, cold icy blue like the Arctic. His hands, giant battered mitts that bunched into fists the size of footballs.”

How about Tom Cruise, whose casting in Friday’s “Jack Reacher” outraged some Child fans, who know their hero as a six-foot-five, 250-pound lug capable of killing someone with a single punch? Cruise is five-foot — well, you’ve seen those photos of him next to Katie Holmes.

Facebook is now home to multiple “Tom Cruise is not Jack Reacher” pages; one with more than 7,500 “likes.” “Tom Cruise playing Jack Reacher is like getting a chimpanzee to play King Kong, an iguana to play Godzilla or a goldfish playing Jaws,” wrote a dissenter named John Clayton Colella. “Sure they can act like those animals, but I’m not scared or a believer.”

“Right from the start, I understood that we were not going to replicate the book’s Reacher in terms of physique, simply because there are no actors — literally none — that are the size of Reacher,” Child tells The Post. “I knew straight away that we were not going to get the externality of Reacher, and that made it all the more important that we got the internality faithfully reproduced.”

Cruise says he’s sensitive to the criticism.

“This is Lee’s book and Lee’s character,” he told Empire magazine. “Him giving me his blessing is what made me do it. If he hadn’t then I wouldn’t have done it.”

Child says that Cruise, who’s known for his charm and glad-handing, has a lot more in common with the stoic drifter character than one might think.

“Tom is a loner,” Child says. “He’s isolated because of his celebrity. He lives a lonely life, fundamentally, and he’s a tough, honest thinker. He does the job in the same way Reacher does. He’s a hard worker and will do what needs doing.”

One thing he’s not, of course, is a homeless wanderer, one of Reacher’s oddest characteristics.

“That was wish fulfillment in a way,” Child says. “Over the last four years with the financial crisis, we’re learning that we don’t own things — things own us. The idea of walking away with absolutely nothing, no commitments, nothing to burden you, complete freedom, that’s a really powerful fantasy right now.”

The fantasy proved irresistible to writer-director Christopher McQuarrie, who is bringing the literary character to the screen. He based the film on “One Shot,” one of the 17 Reacher novels. It finds the drifter traveling to Pennsylvania after one of his former Army colleagues is accused of killing five people in a mass shooting.

Child, a Brit expat who lives and works on East 22nd Street, says three or four attempts to adapt the novels have collapsed over the years.

“You look at the books and think it’s a total no-brainer,” he says. “But at second glance, there are difficult problems with them. A lot of Reacher’s appeal is internal, the quirky thought processes in his head, his eccentricities. How do you get that out on the screen?”

Reacher also doesn’t have an arc.

“Hollywood thinks of characters as going on a journey, being different at the end, learning something along the way,” Child says. “The whole point of Reacher is that he doesn’t change. He doesn’t learn anything, because he knew it all to begin with.”